It is known in the prior art of extruding tire components that it is difficult to extrude tire components featuring a large change in gauge along their cross-sectional area. The difficulty is further increased when the component does not exhibit any plane of symmetry. Such tire components are for instance apexes or wedges. The underlying problem is that the stream of rubber compound exhibits a higher velocity for the rubber compound flowing through the large portion of the cross-sectional opening of a die than through the smaller portion of the cross-sectional opening of the die. Thus, the extruded tire component may be twisted or bent after extrusion. For example, this occurs in case of extruding an apex. This component features a thick edge and a thin edge; hence, material being extruded chooses the path of least flow resistance and therefore converges where the opening of the die is large. This results in an excessive amount of material around the thick edge, a poor feeding around the thin edge and a big difference in the velocity field in the vicinity of the die. Thus, the extrudate exhibits large distortions and the tendency to bend as shown in FIG. 1 which represents schematically the flow of rubber through a prior art extrusion assembly 6 with the bent and distorted apex component 5 being extruded through the space delimited by the die contour 4.
Some proposals to enhance the extrusion of tire components featuring a large change in gauge across their cross-sectional area can be found in the prior art:
GB-A-661,556 mentions the possibility of providing escape openings aside small portions of the cross-sectional area of the die opening which help to obtain a more uniform velocity field of rubber compound flow through the whole cross-sectional area of the die opening. Unfortunately, excessive amounts of rubber compound material are produced which have to be recycled expensively later.
Another possibility which may enhance the uniformity of rubber compound flow is the utilization of a pre-former. A pre-former is positioned in the flow path of rubber compound directly upstream from the die opening, such that the flow velocity of the rubber compound is influenced appropriately to enhance the uniform extrusion of the tire component.
For example a die (downstream view on left hand side) and pre-former (upstream view on right hand side) assembly for the so-called Nakata extruder is shown in FIG. 2. As it can be seen this pre-former 11 is shaped to improve the flow towards the die opening 14. The thickness of the die contour is customized to be thin where the die opening 14 is thin, and is thick where the die opening is thick. In other words, this pre-former 11 renders the flow of the rubber compound easier in portions of the die opening being narrow. The disadvantage of such a solution is the geometrical complexity of the assembly, as each combination of die and pre-former must be customized.
In particular, for the extrusion of a thick wedge often pre-formers as shown in FIG. 3 are used. FIG. 3 illustrates the flow a rubber compound through a prior art extrusion assembly comprising the space delimited by a pre-former 21 and a die contour 24. This pre-former 21, which is placed in the flow upstream from the die opening 24 and downstream from the feeding channel 26, mainly features a cross-section which has an inverted shape with respect to the die opening's shape 24. Therefore indeed rubber compound is pushed laterally towards the edges of the die, and the velocity field is more uniform at all portions of the die opening's cross section. Nevertheless the pre-former 21 constitutes a severe flow restrictor, therefore it is sometimes even called a “brake”. The disadvantage of such a solution is the large amount of viscous heating being produced causing a disadvantageous temperature increase of the extrudate. Overall the pressure drop increases and the total velocity of the rubber compound is significantly disturbed leading to a considerably reduced throughput.
GB-A-661,556 teaches also the use of an ogive shaped deflector to influence the velocity of rubber flow through a channel in front of a die opening for extruding a tire tread. The surface of this deflector has a tapering shape disposed with its smaller end remote from the die and therefore facing the flow of the material to be extruded. The obvious disadvantage of such a deflector is that it is not suitable for many shapes of die openings such that it is not possible to adjust the flow velocity of the rubber compound when extruding for example a wedge or an apex.